For driving a liquid crystal display several DC voltage levels have to be provided to supply the rows and columns of a matrix of pixels of the display with an appropriate electric power. In order to provide various different voltage levels a resistance-dividing scheme or a driver circuit comprising at least one charge pump are known. Document U.S. Pat. No. 6,975,314 B2 describes a display driver having a battery and having numerous regulators whose input is connected to a positive electrode of the battery and whose VSS is connected to the negative electrode of the battery. The output voltages provided by the various regulators might be set to 1.5 V, 3.0 V and 4.5 V.
Generation of multiple and different voltage levels from a given battery voltage typically implies generation of a low voltage level of e.g. 1.5 V and to derive higher voltage levels by multiplying the low voltage level with a given factor, e.g. with a factor of 2 or 3.
Especially for mobile electronic devices it is a general aim to reduce power consumption of the electronic display. It is therefore an aim of the present invention to provide an improved driver circuit for an electronic display such as a LCD that requires less electric power and which enables a longer lifetime of a battery. It is a further aim to provide an improved liquid crystal display exhibiting a reduced power consumption and to provide a portable electronic device comprising such a display.
The patent application EP 1 180 762 A2 describes an image display device with a driver circuit. The driver circuit includes a power supply source connected to a voltage regulator to supply a power supply for first and second charge pumps. The first charge pump is provided to generate at output a scanning voltage Vgh and the second charge pump is provided to generate a non-scanning voltage Vgl. Nothing is described concerning the supply of a low reference voltage for the two charge pumps when the voltage level of the power supply drops below a predefined threshold in order to operate at a low voltage, which is a drawback.